Process for the recovery of a polyol from an aqueous solution

ABSTRACT

A process for the separation of a polyol or multiple polyols in admixture with other organic compounds, usually those produced with the polyol, is described. The process uses a distillation in a column ( 11 ) of a cyclic acetal from an aqueous solution which acetal is formed in a reaction mixture of the polyol and an aldehyde or ketone. The polyols, such as ethylene glycol and propylene glycol, are staple articles of commerce with many uses.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] None

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

[0002] None

BACKGROUND OF THE INVENTION

[0003] (1) Field of the Invention

[0004] The present invention relates to a process for the separation ofa polyol from an aqueous solution. The process involves reactivedistillation of the polyol as a cyclic acetal from an aqueous reactionmixture containing other organic compounds, particularly other polyols.In particular, the cyclic acetal is prepared by reaction of a ketone oraldehyde with the polyol along with distillation of cyclic acetal as itis formed from the reaction mixture.

[0005] (2) Description of the Related Art

[0006] There is a need to recover and purify polyols, including glycols,from an aqueous solution. These polyhydroxy compounds are typicallyformed in multistep processes in dilute aqueous solutions, from whichthe polyol(s) must be separated and purified before being used or sold.These processes include, but are not limited to, production of ethyleneglycol and propylene glycol from their respective epoxides, formation ofpropylene glycol from glycerol, and formation of polyols viahydrogenolysis of sugars or sugar alcohols. All of these processesproduce dilute mixtures of organic compounds including the desiredpolyols in the aqueous solution.

[0007] In the presence of acidic catalysts, glycols (or other polyols)react reversibly with aldehydes and ketones to form cyclic acetals. Thereaction is known as acetalization or ketalization. The acetals of thepolyols are far more volatile than the polyols themselves and much lesspolar, making them easily separable from water by distillation. Becausethe acetalization reaction is reversible, glycols and the aldehyde canbe regenerated by acid hydrolysis of the acetal. The glycol can then berecovered and the aldehyde can be recycled. Ion exchange resins (IER)are one class of materials that can effectively catalyze acetalformation and hydrolysis, but mineral acids and other solid acids areeffective as well. The reaction is as follows:

[0008] Of general interest in connection with this type of reaction in anon-cyclic context is Mahajani, S. M. et al., Reactive And FunctionalPolymers 28 29-38 (1995).

[0009] There have been several reports of the reaction of glycols withaldehydes to form cyclic acetals. Tink and coworkers (Tink, R. R., etal., Can. J. Technol., 29, 243 (1951)) have published a series of papersdescribing recovery of aqueous glycerol solution via reactive extractionwith various aldehydes. As disclosed, n-butyraldehyde and cyclohexanonewere promising among the several aldehydes studied and the former wasparticularly selective. They also studied reactive extraction of severalpolyhydroxy compounds including D-sorbitol, adonitol, dulcitol,D-mannitol and ethylene glycol from aqueous solutions. High distributioncoefficients were obtained with reactive extraction. For instance, withn-butyraldehyde the distribution coefficient for glycerol is 8.3, for EGis 5.9 and for D-sorbitol is 788.

[0010] Broekhuis et al. (Broekhuis, R. R., et al., Ind. Eng. Chem. Res.,33, 3230 (1994)) have compared the various routes for the recovery ofpropylene glycol from dilute aqueous solutions via reaction withaldehydes. They studied lower aldehydes, viz. formaldehyde andacetaldehyde, for reactive distillation and extractive reaction for therecovery. They have claimed to achieve 99+% recovery of propylene glycolin a reactive distillation process. One of the present inventors hasreported on the recovery of ethylene glycol from aqueous solution viaacetalization with formaldehyde (Chopade, S. P. and Sharma, M. M., ReactFunct. Polym. 34(1) 37 (1997)) using ion exchange resins as catalysts.

[0011] A search of the patent literature reveals no processes combiningacetalization with reactive distillation of cyclic acetals for polyolseparation. U.S. Pat. No. 5,917,059 to Bruchmann et al. describespreparation of the cyclic acetal compounds, but does not discuss them incontext of a separation scheme for glycol recovery. There are numerouspatents that describe inventions pertaining to acetals, acetalization,and reactive distillation, but none were found that pertained to ascheme for the recovery of polyols, especially from a dilute mixedsolution of polyols, such as a sugar hydrogenolysis effluent.

[0012] Polyhydroxy compounds show a high affinity towards water and eachother because of hydrogen bonding, and separation of these products fromaqueous solution is conventionally done via a multi-column distillationprocess. In order to obtain ethylene glycol (EG) and propylene glycol(PG), water must be distilled off first because it has a lower boilingpoint temperature than the polyols. The energy to distill off water isthe primary reason for the high cost of polyol separation and recovery.Separation of EG and PG (if they are present together) is also costlybecause they have very similar boiling points, so that a large number ofstages and a large reflux ratio, translating to a large distillationcolumn, is required to achieve the required purities. Purification ofglycerol in a simple distillation column without forming poly-glyceridesand decomposition products is impossible. Vacuum distillation, which hashigh operating costs, is the only distillation route for direct glycerolrecovery.

[0013] Another approach for polyol recovery is solvent extraction ofpolyols from water. Glycols and glycerol have high affinity towardswater (again because of hydrogen bonding), and it is difficult to find asuitable solvent with good distribution coefficient and low miscibilitywith water. Further, extraction only eliminates distillation of largeamounts of water from the product stream. After extraction, there aredistillation steps involving solvent recovery followed by separation ofpolyols from each other. Thus extraction is similar to distillation,except that water is replaced by a solvent.

[0014] There is a need for a safe and effective process for theproduction of polyols. In particular there is a need for a process toefficiently separate EG and PG from aqueous solutions.

Objects

[0015] It is therefore an object of the present invention to provide aneconomical and efficient process for the separation of at least onepolyol from water. It is further an object of the present invention toprovide a process which is relatively easy to perform on a large scalesuitable for commercial production of polyols such as EG and PG. Theseand other objects will become increasingly apparent by reference to thefollowing description and the drawings.

SUMMARY OF THE INVENTION

[0016] The present invention relates to a continuous process forpreparing at least one acetal from an aqueous solution containing atleast one polyol and at least one other organic compound whichcomprises:

[0017] (a) reacting in a combination reaction and distillation vessel areaction mixture of the polyol and an aldehyde or ketone containing 1 to4 carbon atoms in the aqueous solution in the presence of an acidcatalyst, wherein the reaction mixture is introduced into the reactionvessel containing the catalyst with a molar excess of the aldehyde orketone over the polyol to produce the cyclic acetal in the aqueoussolution; and

[0018] (b) separating at least one cyclic acetal from the reactionmixture by distillation.

[0019] Further, the present invention relates to a continuous processfor recovering a polyol from an aqueous solution containing otherorganic compounds which comprises:

[0020] (a) reacting in a combination reaction and distillation vessel areaction mixture of the polyol and an aldehyde or ketone containing 1 to4 carbon atoms in the aqueous solution, wherein the reaction mixture iscontinuously introduced into the vessel containing the catalyst with amolar excess of the aldehyde or ketone over the polyol to produce acyclic acetal in the aqueous solution;

[0021] (b) separating the acetal from the mixture at elevatedtemperatures; and

[0022] (c) hydrolyzing the cyclic acetal produced to recover the polyolas a liquid and the acetaldehyde or ketone which is separated as a vaporfrom the polyol.

[0023] Preferably the reaction mixture is at a temperature, less thanthe boiling point of the reaction mixture, at which at least thealdehyde or ketone is distilled from the reaction vessel as adistillate. Also, preferably if there is more than one cyclic acetalproduced, the cyclic acetals are separated before the hydrolysis step isperformed. The separation can be accomplished in the reaction vessel forthe reactive distillation or in a separate vessel connected to thereaction vessel. Typically the reaction vessel(s) is a heated column.Preferably the desired cyclic acetal is also distilled from the reactionvessel and separated from the aldehyde or ketone.

[0024] Preferably the reaction mixture is at a temperature, less thanthe boiling point of the reaction mixture, at which at least thealdehyde or ketone is distilled from the aqueous solution as adistillate. Preferably the ketone or aldehyde is recycled.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a schematic front view of a polyol recovery system 10with a single reactive distillation column 11 for forming the cyclicacetal(s). EG is ethylene glycol, PG is propylene glycol, and Gly isglycerol.

[0026]FIG. 2 is a schematic front view of the polyol recovery system 10including reactive distillation column 11 and acetal hydrolysis andpolyol recovery columns 18 and 19. MD is 2-methyl-1,3-dioxolane (acetalof EG), DMD is 2,4-dimethyl-1,3-dioxolane (acetal of PG).

[0027]FIG. 3 is a schematic view of reactive distillation column 21 andacetal hydrolysis and polyol recovery columns 25 and 26 as analternative to the process of FIG. 2 for the recovery of EG and PG.

[0028]FIG. 4 shows the experimental column for Example 2.

[0029]FIG. 5 is a graph showing a binary T-x-y vapor liquid-liquidequilibrium (VLLE) diagram for 2MD and water.

[0030]FIG. 6 is a binary T-x-y VLLE diagram for 24DMD and water.

[0031]FIG. 7 is a graph showing vapor pressure data for DMD and MD atvarious temperatures. The data of FIGS. 5 to 7 was developed for thepresent invention.

[0032]FIG. 8 is a graph showing a plot of relative volatility of MD/DMDvs temperature.

[0033]FIG. 9 is a process schematic showing two columns 100 and 101 foracetal formation in column 100 and for acetal recovery in column 101.The column 101 for recycle of acetaldehyde is on the right.

[0034]FIG. 10 is a schematic diagram similar to FIG. 2, except that thePG and EG are separated by distillation in column 30 after hydrolysis ofthe mixed acetals in column 17.

[0035]FIG. 11 is an additional view of the hydrolysis portion of theprocess, shown as vessels 17, 18 and 19 shown in FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0036] The present invention relates to a process involvingacetalization and reactive distillation of a polyol that takes advantageof the reversible reaction of the acetal which is formed and facilitatesthe separation and recovery of the polyol from an aqueous solution. Thefocus of the invention is on recovering ethylene and propylene glycolsfrom aqueous solutions containing higher polyols, but the method hasbroader applications to recovery of a wide range of polyols from water.Boiling points of some acetals of interest for this process are given inTable 1. TABLE 1 Boiling points of some acetals and aldehydes GlycolAldehyde Acetal b.p., ° C. EG Formaldehyde 1,3-dioxolane 74-75 PGFormaldehyde 4-methyl-1,3- 84 dioxolane Glycerol Formaldehyde Glycerolformal 191-195 EG Acetaldehyde 2-methyl-1,3- 82-83 dioxolane PGAcetaldehyde 2,4-dimethyl-1,3- 91-93 dioxolane EG Acetone2,2-dimethyl-1,3- 92-93 dioxolane PG Acetone 2,2,4-trimethyl- 98-991,3-dioxolane Glycerol Acetone 188 Formaldehyde 101 Acetaldehyde 21Isobutyr- 64 aldehyde Acetone 56

[0037] The reactive distillation process involves several distillationcolumns where the acetals are formed, separated from water, andsubsequently hydrolyzed back to the desired polyol. Theacetalization/reactive distillation scheme facilitates polyol recoveryand purification at lower cost than conventional distillation orextraction methods. For the recovery of ethylene glycol and propyleneglycol, acetals formed with formaldehyde or acetaldehyde have lowerboiling points than water, so they can be removed once formed withouthaving to boil off all the water present. This offers a large costsavings over conventional distillation. Further, these acetals have muchlower boiling points than acetals of higher polyols such as glycerol,sorbitol and xylitol, which can be in potential feedstocks for EG/PGproduction, and acetals of process byproducts such as C₄ and C₅ polyols,so they are easily separated from the higher polyols and their acetalsin solution. This aspect of the invention distinguishes it from the workof Broekhuis et al (1994) and Chopade and Sharma (1997), who did notconsider recovery of EG and PG from mixed polyols. Further, the acetalsof EG and PG can be separated from each other at much lower temperaturesand potentially more easily than EG and PG themselves, so the cost ofthe polyol separation is substantially lower as well. Overall, theintegration of the acetalization scheme into a biomass-based polyolsprocess enhances the commercial usefulness of the process.

[0038] Ethylene glycol and propylene glycol are large-scale commoditychemicals: EG is produced at rate of 17 billion lb/yr and PG at about 1billion lb/yr. The present invention provides a more energy efficientroute for EG and PG recovery than the conventional distillation methods.This invention can be combined with the biomass-based production of EGand PG via sugar and sugar alcohol hydrogenolysis to provide aneconomical, renewable resource-based route to EG and PG production.

[0039] The preferred embodiment of the present invention is the recoveryof EG and PG from a mixed polyols stream resulting from hydrogenolysisof sugars or sugar alcohols to polyols. The mixed polyol stream fromhydrogenolysis contains, in addition to EG and PG, glycerol, unreactedfeed (C₅ or C₆ sugar alcohol), and other organic compound byproductssuch as C₄ polyols and lactic acid. Unreacted feed and byproducts arecollectively referred to as “other organic compounds” hereinafter.

[0040] The choice of which acetal to form, e.g. which aldehyde or ketoneto use to acetalize the diols to be recovered, has a strong affect onthe configuration of the process. Formaldehyde was used in the initialfeasability study. However, formaldehyde is a nuisance to theenvironment. Acetals can be formed with a large number of carbonylcompounds, but only acetaldehyde, formaldehyde, and acetone give acetalsof PG and EG which have boiling points below that of water. Thus thepreferred chemical for recovery of EG and PG is acetaldehyde. Theboiling point of acetaldehyde is 21° C. and hence a closed system with agood chilling facility is required. The reactions can be carried outunder pressure, if necessary, to enhance the acetaldehyde concentrationin the liquid phase.

[0041] In reactive distillation, the potential exists for multiplereactions to take place in a single distillation column or, if desired,in multiple distillation columns. The use of a single column can lead tosubstantial savings in capital as well as operating cost. A schematic ofa single column 11 reactive distillation process is shown in FIG. 1.FIG. 1 shows the system 10 including the reactive distillation column11. The column 11 is divided into four sections to help understand theprocess concepts. The sections are the acetalization section 13, theenriching section 12, the hydrolysis section 14 and the strippingsection 15. Sections 12 and 15, the enriching and stripping sections,respectively, are non-reactive, and there is no catalyst in thesesections. Sections 13 and 14 are the reactive sections, which are thekey sections in the process. The aqueous solution containing EG, PG,glycerol, and other products is fed at the top of section 13.Acetaldehyde is fed at a point where sections 13 and 14 meet.Acetaldehyde, being the most volatile component, moves up thedistillation column through section 13. The aqueous feed moves downsection 13 and reacts with acetaldehyde, forming acetals. The acetals ofEG and PG, being more volatile than water, move up the column intonon-reactive section 12. Section 12 strips water from the acetals andthe acetals of EG and PG exit at the top of the column along withunreacted acetaldehyde.

[0042] Because there is essentially no acetaldehyde in section 14, theacetals of glycerol and other organic compounds will hydrolyze in thepresence of catalyst back to glycerol and other organic compounds andacetaldehyde. Any acetaldehyde released will quickly move up the column11 and glycerol and other products are recovered at the bottom of thecolumn 11.

[0043]FIG. 2 shows the column 11 in a reactive distillation system 10with the separation of MD (2-methyl-1,3-dioxolane, which is the acetalof EG) and DMD (2,4-dimethyl-1,3-dioxolane, the acetal of PG). Flashdrum 16 allows the separation of MD and DMD from acetaldehyde which isrecycled. Column 17 separates 2MD from DMD. Recovery column 18 is forhydrolysis of MD and recovery column 19 is for hydrolysis of DMD. Vessel20 is a demineralization vessel to remove inorganic compounds (salts,for instance) if present in the feed stream. Thus, in FIG. 2,acetaldehyde is separated from the acetals in the small column or flashdrum 16 and is recycled back to the acetaldehyde feed to the column 11.The acetals are then separated from each other in the second column 17.Finally, each acetal is separately hydrolyzed back to its correspondingglycol (EG or PG) in the columns 18 and 19, and acetaldehyde is recycledback to column 11.

[0044]FIG. 3 shows an alternate scheme where the acetals are recoveredat the bottom of the reactive distillation column 21. The potentialadvantage of this scheme is that only a stoichiometric quantity ofacetaldehyde would be needed for the acetalization. Acetals areseparated in a second column 22 and then again separated from each otherand subsequently hydrolyzed. The acetaldehyde is refluxed. Column 22 isfor the hydrolysis of polyols other than 2MD, DMD (or other polyolcomponents) with the separation of MD and DMD from acetaldehyde by aflash unit or column 23. The DMD is separated from MD in a multi-stagecolumn 24 and the MD and DMD are separately hydrolyzed in recoverycolumns 25 and 26 to produce ethylene glycol and propylene glycol athigh purity.

EXAMPLE 1

[0045] This Example uses a semi-batch reactor to show the feasibility ofthe reactive distillation scheme for separation of EG and PG fromaqueous solutions. A typical composition of a product stream from a C₅sugar alcohol hydrogenolysis reactor was chosen for these studies. Asshown in Table 2, with formaldehyde as an acetalization agent in 50%excess, 98% recovery of EG and more than 99% recovery of PG wasachieved. However, formaldehyde is less desirable as an acetalizationchemical as previously discussed. TABLE 2 Recovery of EG and PG viasemi-batch acetalization and reactive distillation Initial Finalconcentration in concentration solution (wt %) (% removal in solution inparenthesis) Species (wt %) Run #1 Run #2 EG  7.81 1.64 (79%)  0.16(98%) PG 11.04 0.23 (98%) ˜0.0 (99+%) Glycerol  1.54 NA NA Xylitol  4.62NA NA Total 25.00

EXAMPLE 2

[0046] This Example shows the system used for separation andquantification of the products.

[0047] Distillation column: As shown in FIG. 4, the column 40 consistedof a 2″ (5.08 cm) diameter×7 ft (213.4 cm) tall Pyrex tube in 3′ (36.25cm) and 4′ (121.92 cm) sections. The column 40 contained Katamaxstructured packing 41 from Koch-Glitsch, Ltd. (Wichita, Kans.); up to 15elements were placed into the column for a total packed height of 82″(208.28 cm). The catalyst, a 1 mm ion exchange resin in acid form(Amberlyst 15™, Rhome & Haas), was contained in folded pouches insideeach element. The column 40 was wrapped with heating tape (not shown) intwo-foot sections; the temperature of each heating tape was controlled,with a surface thermocouple and Omega controller to near the internalcolumn 40 temperature to minimize heat losses. At the bottom of thecolumn reboiler 42 consisted of a 1000 ml round-bottom flask 43 held ina heating mantle 44; the reboiler 42 had an overflow level control tomaintain a constant inventory in the reboiler flask. A glass refluxsplitter 45 with a reflux condenser made up the top of the column 40;electronic timers control the reflux ratio at the desired value. Thecondenser was cooled by a 40 wt % solution of ethylene glycol circulatedthrough a chiller to allow condenser temperatures as low as −20° C. Twofeed pumps F1 and F2 dispense feed solutions to the column at acontrolled rate from 1 to 200 ml/min. The column itself had 14 portsthat allowed temperature measurements, introduction of feed, or samplewithdrawal. The column had sections 12, 13 and 15 as shown in FIG. 1.

Analytical Method

[0048] Analysis techniques used liquid chromatography (HPLC) and gaschromatography (GC) for analysis of glycols and acetals. An improved GCmethod was the use of a slightly polar Porapak R packed column (6′×⅛ OD)that enabled the separation of glycols, acetals, water and acetaldehydein one injection. The analysis was conducted in a Varian 3700 gaschromatograph equipped with a thermal conductivity detector and usinghelium carrier gas at a flow rate of 0.45 cm³/s. Column temperature wasinitially maintained at 140° C. for 2 minutes and then increased to 230°C. at a ramp rate of 45° C. /min. The injector and detector blocktemperatures were maintained at 230 and 250° C., respectively. Thismethod allows separation and quantification of water, acetaldehyde, 2MD,24DMD, EG and PG. HPLC was used for analysis of glycols and otherorganic compounds produced in the bottom streams of the distillation. ABio-Rad HPX-87H column with 0.005 M H₂SO₄ as a mobile phase, 50° C.column temperature, and refractive index detection was used.

EXAMPLE 3

[0049] Reaction equilibrium: Batch studies were carried out at severalreaction temperatures to determine the reaction equilibrium for glycolsrecovery using acetaldehyde.

EG+acetaldehyde=2-methyl-1,3-dioxolane (2MD)+water

PG+acetaldehyde=2,4-dimethyl-1,3-dioxolane (24DMD)+water

[0050] The equilibrium constant for these reactions are given as

K_(e)=C_(2MD)C_(H2O)/C_(EG)C_(acetaldehyde)

K_(e)=C_(24DMD)C_(H2O)/C_(PG)C_(acetaldehyde)

[0051] where C is the concentration of the species in the reactionsolution. The results of these experiments are given in Table 3 as afunction of temperature. Also given in Table 3 is the equilibriumconstant for the reaction of PG with acetone, taken from a paper byChopade (Chopade, S., Reactive and Functional Polymers, vol. 42, p201(1999)). TABLE 3 Reaction equilibrium constants for acetal formationEG-Acetaldehyde PC-Acetaldehyde PG-Acetone T(° C.) K_(e) T(° C.) K_(e)T(° C.) K_(e) 25 6.1 25 18.2 30 0.3 44 4.9 40 17 40 0.24 82 3.8 59 13.882 8.4

EXAMPLE 4

[0052] This Example describes VLLE data for two acetals and water, andvapor pressure data for the acetals. These thermodynamic data areimportant for determining the efficacy of the process.

[0053] Thermodynamic data for acetals and acetal/water mixturesVapor-Liquid-Liquid Equilibrium (VLLE): An Othmer still, traditionallyused for the collection of vapor-liquid equilibrium data (Othmer, D.,Ind. & Eng. Chem. 20 743 (1928)), was used to facilitate the collectionof VLLE data for the systems 2MD-water and 24DMD-water. The pure acetalsused in the experiments were prepared by batch processing as follows:excess aldehyde was added to EG or PG, stirred in the presence of ionexchange resin for several hours, and then distilled to recover theacetal-water azeotrope. This azeotrope was then dried using molecularsieves to remove all water present. To determine VLLE data, specifiedquantities of acetal and water were placed in the Othmer still andbrought to reflux. After steady state was reached, as evidenced bycontinuous reflux of the condensed vapor back into the still pot and aconstant liquid temperature, samples of each liquid phase and condensedvapor were taken and analyzed as described above. The diagrams are thusgenerated by changing the mole fraction in the initial charge to thestill across the entire composition range from zero to one.

[0054] The T-x-x-y diagrams for 2MD-water and 24DMD-water are given inFIGS. 5 and 6, respectively; these diagrams contain both theexperimental data and the fit of the data as described below. Water andacetals are only partially miscible, so there are regions where twoliquid phases are present (thus vapor-liquid-liquid information isrequired). In addition to the presence of two liquid phases, bothacetals form minimum-boiling azeotropes with water. It is thisminimum-boiling azeotrope that makes the technology especiallyattractive, as the lower boiling point at which the acetal can berecovered is advantageous.

[0055] Although the phase equilibrium is somewhat complex, it ispossible to take advantage of these complexities to induce moreefficient separations than would otherwise be possible. The VLLE dataare important for understanding experiments, conducting process design,and modeling and conducting economic assessment of the technology.

[0056] The VLLE data for the acetal-water systems were fit to theUNIQUAC equation of state in order to facilitate more practical use ofthe data. The outputs from data regression include the estimatedvapor-liquid data using UNIQUAC and the UNIQUAC binary interactionparameters. The estimated vapor-liquid data must be a close fit to theexperimental vapor-liquid data in order to be of any use. FIGS. 5 and 6show the comparison between the estimated and actual experimentalequilibrium data for the systems of 2MD-water and 24DMD-water. Table 4gives the UNIQUAC binary interaction parameters for each system. TABLE 4UNIQUAC Binary Interaction Parameters Component I DMD 2MD DMD ComponentJ Water Water Actaldehyde Temperature K K K A_(IJ) −14.68 64.31 −0.1225A_(JI) −56.15 −80.00 −0.1708 B_(IJ) 3871.28 2305.94 244.763 B_(JI)−3981.73 −1339.78 450.817 C_(IJ) −2.69 −15.769 −0.0280 C_(JI) 16.9718.214 −0.0746 D_(IJ) 0.0495 0.0567 0.000045 D_(JI) −0.0904 −0.0645−0.004122

[0057] Vapor pressure data: As with the acetal-water VLLE data, vaporpressure data are needed to assess the separation of the acetals and forsimulation studies (such as the UNIQUAC fitting of VLLE data describedabove). Vapor pressure data for pure acetals were collected in a closedpressure vessel. Initially, a small quantity of pure acetal was placedin the vessel and the vessel was placed in an ice bath. When it wascooled, vacuum was applied to remove air, but not strongly enough toboil off the acetal. The initial pressure was noted, and then the closedassembly was put in a constant temperature water bath and allowed toequilibrate. The final pressure was recorded; the difference between theinitial and final pressure is the vapor pressure at that temperature.The experiment was repeated at a number of temperatures.

[0058] The experimental vapor pressure data are shown in FIG. 7. Theconstants in Antoine's equation, which is the standard form used tocharacterize vapor pressure data, were calculated from the aboveexperimental data. Further, the heat of evaporation was calculated fromthe vapor pressure data using the Clausius-Clapeyron equation. TheAntoine's constants, predicted boiling point, and predicted heat ofevaporation are given in Table 5. The predicted values agree veryclosely with the experimental values, thus verifying the accuracy of theexperimental data.

Table 5

[0059] Predicted Antoine's constants and heat of vaporization of 2MD and2,4-DMD a) Antoine's constant Dioxolane A B C Range (C) MD −19.675115909.9 −688.602 25-80 2,4-DMD −4.98443 3238.18 −369.988 25-90

[0060] b) Boiling points Predicted Boiling Point (C.) Dioxolane Boilingpoint (C.) Antoine's eqn. Exponential graph MD 82-83 83.8 83.6 2,4-DMD92 91.26 91.8

[0061] c) Predicted heat of vaporization by Clausius-Clapeyron equationsExpt NIST Std Source calculated (Reported) (Reported) Acetal −Hv/R (K) BHv(KJ/mol) KJ/mol KJ/mol 2MD −4230.60 89.033 35.17322 35 34.32 2,4-DMD−3684.16 62.33 30.63007 NA NA

Separation of Acetals by Conventional Distillation

[0062] The ratio of the vapor pressures of the two acetals (from FIG. 7)is equal to the relative volatility of the 2MD to 24DMD if mixtures ofthe two are considered ideal. The plot of temperature versus relativevolatility is shown in FIG. 8. At the temperature range over whichseparation of the two species would take place at atmospheric pressure(80-90° C.), the value of the relative volatility of 2MD and 24DMD isabout 1.3. Thus, separation of 2MD from 24DMD is possible by fractionaldistillation.

[0063] An experiment to illustrate the separation of the two acetals wascarried out in a small distillation column. The column consisted of a 1¼″ diameter×5 ft tall Pyrex tube packed with wire mesh packing similarto FIG. 4. The column is wrapped with heating tape and the temperatureof heating tape is controlled (with a surface thermocouple and Omegacontroller) to near the internal column temperature to minimize heatlosses. The reboiler consists of a 500 ml roundbottom flask held in aheating mantle; the reboiler has an overflow level control to maintain aconstant inventory in the reboiler flask. A glass reflux splitter with areflux condenser makes up the top of the column; electronic timerscontrol the reflux ratio at the desired value. The condenser is cooledby a 40 wt % solution of ethylene glycol circulated through a chiller toallow condenser temperatures as low as −20° C. The feed pumps dispensefeed solutions to the middle of column at a controlled rate from 1 to200 ml/min. The column has 5 ports that allow temperature measurements,introduction of feed, or sample withdrawal.

[0064] The experiment was conducted by placing an equimolar mixture of2MD and 24DMD into the reboiler and then bringing the column totemperature at total reflux. At this condition, 99+% pure 2MD wasobtained at the top of the column and 99+% 24DMD was obtained at thebottom of the column, respectively. This clearly indicated thefeasibility of separation of these acetals in a single column.

EXAMPLE 5

[0065] Continuous reactive distillation: FIG. 9 shows the two columns100 and 101 used in Example 2. Column 100 is for the reactivedistillation and column 101 is for the separation of the acetals fromthe acetaldehyde. Vessels 102 and 103 are reflux condensers and vessel104 is the reboiler for vessel 100. Vessel 105 is a reboiler for vessel101. This system allows for recycling acetaldehyde while maintainingfavorable acetaldehyde and glycol molar ratios in column 100. Becauseacetaldehyde is so volatile and somewhat difficult to handle, atwo-column system 100 and 101 was used for acetaldehyde-glycol studiesto recycle acetaldehyde through the reactive distillation column 100.This system, shown schematically in FIG. 9, allows maintenance of highratios of acetaldehyde to glycol (up to 10:1) in the column 100 whilenot consuming large quantities of acetaldehyde. Experiments wereconducted to demonstrate acetal recovery in a reactive distillationcolumn described in Example 2. The system PG-acetone was used initiallyfor shakedown and to develop a familiarity with column 100 operation,because acetone has a boiling point of 58° C. (as opposed toacetaldehyde, which boils at 21° C.) and is easily handled at roomtemperature.

[0066] Results of the continuous reactive distillation experiments aregiven in Table 6. TABLE 6 Results of continuous reactive distillationexperiments Glycol Glycol feed Glycol Reflux Height of concen- Glycolconcen- solution ratio in catalyst tration in conversion tration feedrate column section bottoms to acetal System (Wt %) (g/min) (L/D) (in.)(wt %) (%) PG-acetone 100 3.6 0.25 39 83 83 PG-acetone 75 3.6 0.25 39 6347 EG-acetaldehyde 50 3.6 1 28 16 55 EG-acetaldehyde 50 6.0 1 28 29 42PG-acetaldehyde 50 3.6 1 28 0.6 99 PG-acctaldehyde 25 3.6 0.75 28 1.4 94

[0067] The column operated as expected in initial shakedown runs usingPG-acetone, but the formation of the acetal,2,2,4-trimethyl-1,3-dioxolane, was substantial only at highconcentrations of PG (50% -100%) in the feed solution. These initialruns demonstrate that the ion exchange resin is active for acetalformation.

[0068] The experiments with EG-acetaldehyde and PG-acetaldehyde clearlydemonstrate the feasibility of glycol recovery via acetal formation. Inthe run using 50% PG feed solution in water, 99% of PG is removed fromthe aqueous feed stream, leaving a bottoms product of essentially purewater. The recovery of EG is lower than for PG, corresponding to thelower reaction equilibrium conversion of EG to 2MD. The potential forincreasing all recoveries is excellent, as these experiments werecarried out with only 28″ (71.12 cm) of catalyst section and moderateacetaldehyde recycle rates. Longer catalyst section would allowdemonstration of nearly complete EG recovery.

[0069] The results compiled demonstrate the reactive distillation andrecovery of polyols from aqueous solution.

EXAMPLE 6

[0070] This example shows a mixed feed solution of sorbitol, glycerol,EG, and PG. Only the acetals of EG and PG are removed in the top of thecolumn, along with acetaldehyde. The bottoms consist of unreacted EG,PG, glycerol, sorbitol, and acetals of glycerol and sorbitol. Theresults are shown in Table 7. TABLE 7 Acetalization of simulatedsolution *Table shows only top composition in mol % Feed: 15% PG + 7%EG + 5% Glycerol + 5% Sorbitol (wt %) Molar Feed Feed Feed rate, feedratio, Conv % Bottom Position Position AcH Feed: Reflux PG Topcomposition, mol % Composition, Temperature profile, C Run AcH(Solution) Mol/min AcH ratio (EG) H2O AcH MD DMD mol % Top MiddleBottom 1. F2 F1 0.101 1/6 1:4 52.28 25.94 61.23 2.33 10.50 Mixture of 4868-101 101- 18.47 unreacted feed 103 2. F2 F1 0.119 1/5 1.4 34.20 49.1442.24 1.72 6.89 and their 65 85-101 101- 11.94 acetals of high 103boiling points (PG: 0.42%) (EG: 1.2%)

EXAMPLE 7

[0071] This Example shows the results with a feed solution containing amixture of EG, PG and other polyols. There was no hydrolysis in section14 of FIG. 1. The results are shown in the acetals of EG and PGproduced. Small quantities of 4-ethyl-2-methyl-1,3-dioxolane (EMD), theacetal of 1,2-butanediol, were present at the top of the distillationcolumn. The concentration of EMD will be reduced in a taller,commercial-scale system. No glycerol or sorbitol cyclic acetals werefound in the PG or EG produced. The results are shown in Table 8. TABLE8 ACETALIZATION OF MIXED SOLUTION OF POLYOLS Feed: 15% PG + 7% EG + 5%Glycerol + 5% Sorbitol + 2% 1-2 Butanediol (all in wt %) Bottom MolarTop composition, mol % composition, Feed Feed feed From Column 11 mol %Posi- Feed rate, ratio, Re- Conv % EMD From Temperature profile, C tionPosition AcH Feed: flux PG (Acetal of 2- Column In Column 11 Run AcH(Solution) Mol/min AcH ratio (EG) H2O AcH MD DMD butanediol) 11 TopMiddle Bottom 1. F2 F1 0.119 1/6 1:4 53.27 29.31 54.97 3.63 12.04 0.023Mixture of 52 68-101 101-103 25.39 unreacted 2. F2 F1 0.101 1/5 1.427.53 49.14 42.24 1.72 6.89 0.45 feed and 70 82-101 101-103 10.85 theiracetals 3 F2 F1 0.082 1/4 1:4 57.78 21.03 54.96 5.67 18.28 0.03 of high44 65-101 101-103 28.28 boiling 5. F2 F1 0.06 1/3 1:4 54.47 15.32 66.073.5 12.54 2.4 points 50 66-101 101-103 24.61 4. F2 F1 0.119 1/6 1:469.22 14.1 67.58 8.95 29.21 0.09 44 62-101 101-103 31.44 (12) (13) (15)

EXAMPLE 8

[0072] Example 8 shows the fractional distillation of DMD and MDmixtures with and without water. The distillate column was 1 ½″ (3.8 cm)in diameter and 5 ft (152.4 cm) in length with wire mesh packings. Theresults in Tables 9 and 10 show that such separations are feasible.TABLE 9 A) Distillation of mixture of pure components Distillate BottomTemperature composition, composition, profile, Reflux mol % mol % C Runratio MD DMD MD DMD Top Bottom 1 Total 81.56 18.44 0.0 99.999 83 102Reflux 2 Total 91.7 8.3 0.0 99.999 82 97 Reflux

[0073] TABLE 10 B) Distillation in presence of water Distillatecomposition, Bottom composition, Temperature Reflux mol % mol % profile,C Run ratio MD DMD Water AcH MD DMD Water AcH EG PG Top Bottom 1 Total57.17 3.01 39.36 0.00 17.80 82.19 0.0 — — — 75 92 Reflux 2 Total 37.8117.43 33.24 11.5 0.0 87.17 0.0 0.0 6.3 6.4 82 104 Reflux

EXAMPLE 9 Acetal Hydrolysis via Reactive Distillation

[0074] Having demonstrated that the acetals of EG and PG can be formedand recovered via reactive distillation using an ion exchange resin inacid form (Amberlyst 15), the hydrolysis of acetals in the reactivedistillation column to obtain high purity propylene and ethylene glycolwas examined. The emphasis was primarily to have high purity propyleneglycol, specifically with a very low (ppm) level of acetaldehydeimpurity. As part of this effort, process simulation software was usedwith the VLLE data to help design the lab experiments and verify thepotential of obtaining high purity PG.

[0075] Hydrolysis of 2,4DMD: Initial experiments were carried out with a100 cm reaction zone (packing with catalyst) and 200 cm of totalstructured packing (Katamax structured packing, Koch-Glitsch, Ltd.,Wichita, Kans.). Because of substandard performance with 100 cm ofreaction zone, later experiments were conducted with a 140 cm reactionzone and 200 cm total packing. The catalyst, 1 mm ion exchange resinbeads in acid form (Amberlyst 15), is contained in folded pouches insideeach element of the reaction zone. The details of the packing arementioned in the previous Examples.

[0076] The column was operated under steady state at a variety of refluxratios, temperature profiles, and water:acetal feed ratios. Theexperimental results are tabulated in Table 11. TABLE 11 Hydrolysis of2,4 Dimethyl dioxolane Feed Feed Feed rate, Molar feed Distillatecomposition, Bottom composition, Temperature Position Position DMDratio, Reflux Conv % mol % mol % profile, C Run H2O DMD mol/hr DMD:H2Oratio (DMD) H2O AcH DMD H2O DMD PG Top Middle Bottom 1 F2 F 1.99 1/2 1:264.29 15.59 67.87 16.13 69.61 0.00 30.38 42 82-98  98-103 2 F2 F 1.991/2 1:2 72.89 40.51 47.09 12.38 77.70 0.00 21.17 64 86-99 100-106 3 F1 F1.99 1/2 1:2 77.22 17.02 65.59 16.98 79.24 0.00 20.75 56 84-98  99-105 4F1 F 1.99 1/2 1:4 80.13 17.06 66.28 16.65 78,41 0.00 21.58 54 84-98 99-105 5 F1 F 1.99 1/1.2 1:4 81.7 13.12 73.40 13.46 65.12 0.00 34.75 5484-98  99-106 6 F1 F 1.99 1/1.5 1:4 81.11 23.78 63.07 13.13 69.00 0.0030.99 54 84-98  99-106 7 F1 F 1.99 1/3 1:4 79.54 20.88 66.46 12.65 80.090.00 19.05 54 84-98  99-106

[0077] With 100 cm of reaction zone, conversion of 24DMD to PG of about75% was obtained. With the longer reaction zone of 140 cm and a shorterrectifying section, which allows more residence time in the catalyticsection, up to 80% DMD conversion was achieved. Process simulationpredicts and experiments verify that a water:acetal ratio of 1.2 to 2 issufficient; higher water:acetal ratios do not further improve columnperformance. Most importantly, the PG product coming from the bottom ofthe column is very pure as seen in Table 11. TABLE 12 Hydrolysis of 2Methyl dioxolane Feed Feed Feed rate, Molar Distillate composition,Bottom composi- Temperature Position Position MD feed ratio, Reflux Conv% mol % tion, mol % profile, C Run H2O MD mol/hr MD:H2O ratio (MD) H2OAcH MD H2O MD EG Top Middle Bottom 1 F1 F 2.67 1/2 1:4 80.0 21.7 73.904.30 71.04 0.00 28.95 54  98-102 104-108 2 F1 F 2.67 1/3.8 1:4 81.5 15.978.20 5.92 84.14 0.00 15.85 48 96-99 104-108 3 F1 F 2.67 1/2 1:4 91.720.6 65.50 13.80 74.12 0.00 27.87 40 96-99 104-108 4 F1 F 2.67 1/3 1:496.0 17.6 77.95 4.47 77.34 0.00 22.65 40  96-100 104-110 5 F1 F 2.67 1/41:4 93.4 18.06 75.66 6.27 82.43 0.01 16.35 44  96-101 104-108 6 F1 F2.67 1/1.2 1:4 88.3 19.5 75.50 4.99 65.38 0.00 34.61 42  96-101 105-120

[0078] Hydrolysis of 2MD: In experiments parallel to those describedabove for 24DMD, hydrolysis of 2MD was studied. The experimental resultsare summarized in Table 12, and show that higher conversions of 2MD toEG, up to 95%, can be achieved than for 24DMD to PG. TABLE 13 Hydrolysisof mixed acetal (2MD and 2,4 DMD) Molar Feed feed Feed rate, ratio, ConvFeed Point mol/h DMD + Re- % Top composition, Bottom composition,Temperature Point Ace- DMD + MD: flux DMD mol % mol % profile, C Run(H2O) tal MD H2O ratio (MD) H2O AcH MD DMD H2O MD DMD EG PG Top MiddleBottom 1 F1 F 2.24 1/2.5 1:4 75.92 49.82 31.03 2.2 16.96 77.5 0.0 0.09.51 12.97 60 98-102 100-112 91.03 2 F1 F 2.24 1/2.5 1:4 83.7 10.3578.17 1.37 10.10 82.65 0.0 0.001 6.86 10.48 43 97-98   99-107 89.9 3 F1F 2.24 1/2.5 1:4 86.69 13.32 76.14 1.43 9.38 71.36 0.0 0.0 11.15 17.4848 97-99  100-110 90.48 5 F1 F 2.24 1/4.1 1:4 87.8 9.5 82.35 0.5 7.683.15 0.0 0.0 6.5 10.25 43 97-98   99-107 92.4 4 F1 F 2.24 1/1.2 1:478.06 17.42 69.62 1.34 11.55 72.01 0.0 0.0 11.33 16.65 50 98-101 101-12086.96

[0079] This is because the reaction equilibrium for hydrolysis of theacetal of EG is more favorable than that for hydrolysis of the acetal ofPG. The reaction equilibrium constants are tabulated in Table 3.

[0080] The conversion of acetal to glycol in hydrolysis is limited inthese examples by the height of packing available in thelaboratory-scale column. With a longer reactive zone, completehydrolysis of the acetal will take place. This is supported by theprocess simulations set forth hereinafter.

[0081] Alternative mixed acetal hydrolysis scheme: In an alternativescheme for the separation and hydrolysis of the acetals formed inreactive distillation, a mixture of both acetals are first hydrolyzed inone reactive distillation column and then the resulting mixture of PGand EG is separated in a conventional distillation column. This routecan potentially reduce the number of distillation columns required fromthree to two, with only one reactive distillation column versus two inthe original concept. This alternate route involves the separation of EGfrom PG, which is practiced commercially but is a difficult, expensiveseparation. This alternate scheme is shown as columns 29 and 30 in FIG.10.

[0082] The hydrolysis of mixed acetals (2MD and 24DMD) was carried outin the same reactive distillation column as described above forindividual acetal hydrolysis. Experiments were performed with mixturesof acetals only and with the acetals with water; mixture compositionswere chosen to simulate the products from the acetal formation column.The results are tabulated in Tables 13 and 14 as a function of water:acetal ratio and reflux ratio. TABLE 14 Hydrolysis of mixed acetal (2MDand 2, 4 DMD) along with water Over all Feed Feed Feed Molar Conv %Point Point rate, mol/h feed ratio, DMD Top composition, mol % Run (H₂O)Acetal DMD + MD DMD + MD:H₂O Reflux ratio (MD) H₂O AcH MD DMD 6 F1 F2.17 1/2.75 1:4 81.14 20.34 63.20 2.34 14.12 93.68 7 F1 F 2.17 1/3.1 1:484.68 10.93 77.85 1.53 9.68 89.9 Bottom Composition, mol % Temperatureprofile, C. Run H₂O MD DMD EG PG Top Middle Bottom 6 81.86 0.0 0.0 8.339.8 44 97-99 100-110 7 80.53 0.0 0.0 8.5 10.88 46 96-99 100-110

[0083] Up to 95% conversion of 2MD and 85% conversion of 24DMD, valueseven slightly higher than those for the individual acetals, wereachieved at optimum conditions. These results are promising and indicatethat mixed acetal hydrolysis is a viable alternative to the originalconcept of separating acetals prior to hydrolysis.

Process Simulation of Hydrolysis

[0084] Computer process simulation software (Aspen Plus 10.1, Aspentec,Inc.) was used to model the proposed reactive distillation hydrolysisprocess. These simulations were conducted in part to help defineexperimental parameters for developmental studies, and more importantlyto demonstrate the behavior of commercial scale processes particularlyregarding product purities. Process simulation provides a means, basedon experimental findings and thermodynamic (e.g. VLLE) data, to predictwith significant confidence the performance of the proposed separationtechnology at the commercial level. TABLE 15 Simulation results forsingle acetal (2,4 DMD) hydrolysis scheme 1 Feed 2 Feed 3 Distillate 4Bottoms Mole Flow kmol/hr Water 60 0 10 1.28E−04 Acetaldehyde 0 0 50.005.37E−16 Propylene 0 0 2.56E−11 49.99987 Glycol DMD 0 50 1.28E−041.43E−19 Total Flow 60 50 60 50 KMOL/HR Temperature K 298.2 298.2 330.5460.9 Number of Stages 20 Reflux Ratio 3 Boilup Ratio 7 Water FeedLocation 2 DMD Feed Location 7 HETP (m) 0.5 Reaction Zone Stage 5-15Packed Zone Stage 5-15 Packing Kerapak Packing Height (m) 5.5 ColumnDiameter (m) 1.78

[0085] The simulation results of 24DMD hydrolysis to PG, correspondingto column 26 in FIG. 11, are given in Table 15.

[0086] The parallel results for 2MD hydrolysis to EG, corresponding tocolumn 25 in FIG. 11, are given in Table 16. TABLE 16 Simulation resultsfor single acetal (2MD) hydrolysis scheme 1 Feed 2 Feed 3 Distillate 4Bottoms Mole Flow kmol/hr Water 60 0 10 5.57E−08 Ethylene 0 0 7.79E−1750 Glycol Acetaldehyde 0 0 50 5.00E−29 2MD 0 50 5.57E−08 5.00E−29 TotalFlow 60 50 60 50 KMOL/HR Temperature K 298.15 298.15 330.4646 470.2331Number of Stages 25 Reflux Ratio 3.5 Packing Height (m) 8 ColumnDiameter (m) 1.71

[0087] The columns described in Tables 15 and 16 produce approximately60 million lb mol of propylene glycol and 50 million lb mol of ethyleneglycol per year; the column diameters are 1.78 m for the 24DMDhydrolysis column and 1.71 m for the 2MD hydrolysis column. Each columnhas been optimized to reduce the number of equilibrium stages until themaximum allowable amount of water is present in the glycol productstream in accordance with industry standards. Water:acetal feed ratioswere reduced to slightly above the stoichiometric molar ratios to reduceoperating costs of the condenser and reboiler. Inlet stream temperatureswere set at room temperature to mimic experimental conditions. Emphasiswas placed on reducing the level of acetaldehyde in the glycol productstreams to the order of part-per-million levels. It is seen that theacetaldehyde content in all of the glycol product streams is negligible,indicating that very high purity PG and EG with reasonable column sizes(˜25 stages) in a commercial process can be obtained.

[0088] The simulation results of mixed acetal hydrolysis are given inTable 17. TABLE 17 Simulation results for mixed acetal hydrolysis scheme1 Feed 2 Feed 3 Distillate 1 4 Bottoms 1 5 Distillate 2 6 Bottoms 2 MoleFlow kmol/hr Water 83.1586 28.34952 1.70E+00 2.43E−04 2.43E−04 3.69E−35Acetaldehyde 0 0 109.8069 1.01E−14 0 0 Propylene Glycol 0 0 5.30E−1168.22761 68.22689 7.13E−04 Ethylene Glycol 0.00E+00 0 6.02E−15 41.57933.27E−04 41.57897 DMD 68.22785 0 2.43E−04 2.09E−14 0 0 2MD 41.5793 01.21E−08 1.10E−28 0 0 Total Flow KMOL/HR 192.9658 28.34952 111.5081109.8072 68.22747 41.57969 Temperature K. 3.51E+02 3.48E+02 295.39564.64E+02 4.61E+02 4.70E+02 Column 1 Column 2 Number of Stages 30 123Reflux Ratio 5 12 Boilup Ratio 4.51 21.8 Water Feed Location 2 — MixedFeed Location 12 51 HETP (m) 0.5 — Reaction Zone Stage 2-15 — PackedZone Stage 2-20 0.609 m/stg Packing/Trays Kerapak Sieve Packing/ColHeight (m) 9.5 74.9 Column Diameter (m) 2.14 2.8

[0089] This hydrolysis column was optimized in the same manner as thesingle acetal hydrolysis. It also produces approximately 60 million lbmol of propylene glycol and 50 million lb mol of ethylene glycol peryear. The column diameter of the mixed acetal hydrolysis is 2.14 m,somewhat larger than the single acetal hydrolysis columns. The secondcolumn to separate EG and PG via conventional distillation is verylarge. Again, a negligible quantity of acetaldehyde is present in theproduct mixed glycol stream, indicating that pure EG and PG can beproduced in a commercial-scale process.

Summary

[0090] The combined results of experimental findings and simulationstudies show the feasibility of reactive distillation for polyolsrecovery from aqueous solution. In particular, two viable scenarios arepresented here for the separation and hydrolysis of acetals produced inreactive distillation to pure PG and EG. Experimental findings are inaccordance with thermodynamic and reaction data, but experimentalconversion and product purities are limited by the size (particularlythe height) of the laboratory-scale equipment. Simulation studies, basedon experimental data, demonstrate with a high degree of confidence thatthe required product purities and recoveries can be achieved incommercial-scale equipment.

[0091] It is intended that the foregoing description be onlyillustrative of the present invention and that the present invention belimited only by the hereinafter appended claims.

We claim:
 1. A continuous process for preparing at least one acetal froman aqueous solution containing at least one polyol and at least oneother organic compound which comprises: (a) reacting in a combinationreaction and distillation vessel a reaction mixture of the polyol and analdehyde or ketone containing 1 to 4 carbon atoms in the aqueoussolution in the presence of an acid catalyst, wherein the reactionmixture is introduced into the reaction vessel containing the catalystwith a molar excess of the aldehyde or ketone over the polyol to producethe cyclic acetal in the aqueous solution.; and (b) separating at leastone cyclic acetal from the reaction mixture by distillation.
 2. Theprocess of claim 1 wherein there is more than one polyol in the reactionmixture which is reacted.
 3. The process of claim 1 wherein the reactionmixture contains ethylene glycol and propylene glycol as polyols whichreact with the aldehyde in the mixture to form the cyclic acetal.
 4. Theprocess of any one of claims 1, 2 or 3 wherein the reaction mixturecontains glycerol, sorbitol and C4 diols and triols as plural of theother organic compound which react with the aldehyde to form additionalcyclic acetals which have a higher boiling point than the cyclic acetaldistilled from the reaction mixture.
 5. The process of claims 1, 2 or 3wherein in addition the excess aldehyde or ketone is recycled to step(a).
 6. The process of any one of claims 1, 2 or 3 wherein acid catalystin step (a) is an acidic resin.
 7. The process of any one of claims 1, 2or 3 wherein the aldehyde in step (a) is acetaldehyde.
 8. The process ofclaims 1, 2 or 3 wherein the cyclic acetal is distilled from thereaction mixture with the aldehyde or ketone.
 9. A continuous processfor recovering a polyol from an aqueous solution containing otherorganic compounds which comprises: (a) reacting in a combinationreaction and distillation vessel a reaction mixture of the polyol and analdehyde or ketone containing 1 to 4 carbon atoms in the aqueoussolution, wherein the reaction mixture is continuously introduced intothe vessel containing the catalyst with a molar excess of the aldehydeor ketone over the polyol to produce a cyclic acetal in the aqueoussolution; (b) separating the acetal from the mixture at elevatedtemperatures; and (c) hydrolyzing the cyclic acetal produced to recoverthe polyol as a liquid and the acetaldehyde or ketone which is separatedas a vapor from the polyol.
 10. The process of claim 9 wherein there ismore than one polyol in the aqueous solution after step (a) and whereinthe polyols are separated after step (b).
 11. The process of claims 9 or10 wherein in step (a) the cyclic acetal is distilled from the vessel.12. The process of claims 9 or 10 wherein the excess aldehyde or ketoneis recycled to step (a).
 13. The process of claims 9 or 10 wherein theacid catalyst is an acidic resin.
 14. The process of claims 9 or 10wherein the aldehyde is acetaldehyde.
 15. The process of claims 9 or 10wherein the reaction mixture contains ethylene glycol and propyleneglycol as polyols which react with the aldehyde or ketone to form thecyclic acetal.
 16. The process of claims 9 or 10 wherein the cyclicacetal is distilled from the reaction mixture with the aldehyde andwherein the mixture introduced into the reaction vessel containsglycerol, sorbitol and C4 diols and triols as plural of the otherorganic compound which react with the aldehyde to form additional cyclicacetals which have a higher boiling point than the cyclic acetal whichis distilled from the reaction mixture.
 17. The process of claims 1, 2or 3 wherein the reaction mixture contains cyclic acetals of at leasttwo polyols which are separated from the reaction mixture together, thenseparated from each other and then hydrolyzed separately to the polyols.18. The process of claims 8 or 9 wherein the reaction mixture containscyclic acetals of at least two polyols which are separated from thereaction mixture together and then separated from each other and thenhydrolyzed separately to the polyols.
 19. The process of any one ofclaims 1, 2 or 3 wherein the reaction mixture is at a temperature, lessthan the boiling point of the reaction mixture, at which the aldehyde orketone is distilled from the reaction mixture.
 20. The process of anyone of claims 9, 10 or 11 wherein there is a mixture of acetals andwherein the acetals are hydrolyzed together to their respective polyolsand then the polyols are separated.
 21. The process of claims 9, 10 or11 wherein there are a mixture of acetals and wherein the acetals areseparated and then hydrolyzed to form the isolated polyols.
 22. Theprocess of claims 9, 10 or 11 wherein the reaction mixture is at atemperature, less than the boiling point of the reaction mixture, atwhich the aldehyde or ketone is distilled from the reaction mixture.